The Mascall fauna is a well-known middle Miocene (Barstovian) mammalian assemblage in the Pacific Northwest. It has been collected for over 100 years and collecting intensity has increased since the establishment in 1975 of a national monument enclosing the type area of the formation. Despite its importance to biostratigraphy, biogeography of Barstovian taxa, and paleoecological studies, the fauna at the type locality has not been taxonomically examined in more than 50 years. Evaluation and classification of the stratigraphy of the Mascall Formation (Bestland et al. 2008) has prompted a faunal revision in order to place taxa within the new stratigraphic framework. Here we report on the fauna from the type area of the Mascall Formation in central Oregon, and conclude that 20 taxa are new to the fauna, and several taxa previously assigned to distinct species are synonymized. We also place specimens and taxa within a robust stratigraphic framework, calibrated with new U-Pb radioisotopic ages for the Mascall Tuff (15.122±0.017 Ma), the most fossiliferous layer in the formation, and the Kangaroo Tuff (13.564±0.018 Ma), the upper most tuff in the formation.

New geochronologic and paleomagnetic data from the North American Midcontinent Rift (MCR) reveal the synchronous emplacement of the Beaver River diabase, the anorthosite xenoliths within it, and the Greenstone Flow—one of the largest lava flows on Earth. A U-Pb zircon date of 1091.83 (Formula presented.) 0.21 Ma (2 (Formula presented.)) from one of the anorthosite xenoliths is consistent with the anorthosite cumulate forming as part of the MCR and provides a maximum age constraint for the Beaver River diabase. Paired with the minimum age constraint of a cross-cutting Silver Bay intrusion (1091.61 (Formula presented.) 0.14 Ma; 2 (Formula presented.)), these data tightly bracket the age of the Beaver River diabase to be 1091.7 (Formula presented.) 0.2 Ma (95% CI), coeval with the eruption of the Greenstone Flow (1091.59 (Formula presented.) 0.27 Ma; 2 (Formula presented.))—which is further supported by indistinguishable tilt-corrected paleomagnetic pole positions. Geochronological, paleomagnetic, mineralogical and geochemical data are consistent with a hypothesis that the Beaver River diabase was the feeder system for the Greenstone Flow. The large areal extent of the intrusives and large estimated volume of the volcanics suggest that they represent a rapid and voluminous ca. 1,092 Ma magmatic pulse near the end of the main stage of MCR magmatism.

Despite being a prominent continental-scale feature, the late Mesoproterozoic North American Midcontinent Rift did not result in the break-up of Laurentia, and subsequently underwent structural inversion. The timing of inversion is critical for constraining far-field effects of orogenesis and processes associated with the rift’s failure. The Keweenaw fault in northern Michigan (USA) is a major thrust structure associated with rift inversion; it places ca. 1093 Ma rift volcanic rocks atop the post-rift Jacobsville Formation, which is folded in its footwall. Previous detrital zircon (DZ) U-Pb geochronology conducted by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) assigned a ca. 950 Ma maximum age to the Jacobsville Formation and led researchers to interpret its deposition and deformation as postdating the ca. 1090–980 Ma Grenvillian Orogeny. In this study, we reproduced similar DZ dates using LA-ICP-MS and then dated 19 of the youngest DZ grains using high-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS). The youngest DZ dated by CA-ID-TIMS at 992.51 ± 0.64 Ma (2σ) redefines the maximum depositional age of the Jacobsville Formation and overlaps with a U-Pb LA-ICP-MS date of 985.5 ± 35.8 Ma (2σ) for late-kinematic calcite veins within the brecciated Keweenaw fault zone. Collectively, these data are interpreted to constrain deposition of the Jacobsville Formation and final rift inversion to have occurred during the 1010–980 Ma Rigolet Phase of the Grenvillian Orogeny, following an earlier phase of Ottawan inversion. Far-field deformation propagated >500 km into the continental interior during the Ottawan and Rigolet phases of the Grenvillian Orogeny.

During the Cryogenian (720 to 635 Ma ago) Snowball Earth glaciations, ice extended to sea level near the equator. The cause of this catastrophic failure of Earth's thermostat has been unclear, but previous geochronology has suggested a rough coincidence of glacial onset with one of the largest magmatic episodes in the geological record, the Franklin large igneous province. U-Pb geochronology on zircon and baddeleyite from sills associated with the paleo-equatorial Franklin large igneous province in Arctic Canada record rapid emplacement between 719.86 ± 0.21 and 718.61 ± 0.30 Ma ago, 0.9 to 1.6 Ma before the onset of widespread glaciation. Geologic observations and (U-Th)/He dates on Franklin sills are compatible with major post-Franklin exhumation, possibly due to development of mafic volcanic highlands on windward equatorial Laurentia and increased global weatherability. After a transient magmatic CO₂ flux, long-term carbon sequestration associated with increased weatherability could have nudged Earth over the threshold for runaway ice-albedo feedback.

Mafic intrusions, lava flows, and felsic plutons in southwestern Laurentia have been hypothesized to be associated with the emplacement of a late Mesoproterozoic (Stenian Period) large igneous province. Improved geochronologic data resolve distinct episodes of mafic magmatism in the region. The ca. 1,098 Ma main pulse of southwestern Laurentia large igneous province (SWLLIP) magmatism is recorded by mafic intrusions across southeastern California to central Arizona. A younger episode of volcanism resulted in eruptions that formed the ca. 1,082 Ma Cardenas Basalt, which is the uppermost unit of the Unkar Group in the Grand Canyon. With the updated geochronological constraints, we develop new paleomagnetic data from mafic sills in the SWLLIP. Overlapping poles between the Death Valley sills and rocks of similar age in the Midcontinent Rift are inconsistent with large-scale Cenozoic vertical axis rotations in Death Valley. We also develop a new paleomagnetic pole from the ca. 1,082 Ma Cardenas Basalt (pole longitude = 183.9°E, pole latitude = 15.9°N, (Formula presented.) = 7.4°, N = 18). The new paleomagnetic data are consistent with the pole path developed from time-equivalent rocks of the Midcontinent Rift, supporting interpretations that changing pole positions are the result of rapid equatorward motion. These data add to the record of Laurentia's rapid motion from ca. 1,110 to 1,080 Ma that culminated in collisional Grenvillian orogenesis and the assembly of Rodinia.